[swift-evolution] [Pitch] Can we make `default` on switches optional?

[Robert Widmann]

-1 in general.  I want smarter exhaustiveness analysis, but I don’t want to be able to ignore cases that “can’t happen” (so say we, writer of bugs) when they’re clearly in the domain of possible values that can be fed to a switch-case.  Exhaustiveness guarantees wellformedness of a program that does happen to go wrong, and makes it much easier to verify the correctness of the flow of control of the containing block because all points from the switch must be covered.  We also don’t have the type-level tools to convince the checker to allow you to remove unreachable cases.  If it’s really a problem that you are writing default cases everywhere, just bailout in a fatal error with a nice description.  It never hurts.

~Robert Widmann

> On Oct 3, 2016, at 6:14 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I know that there is this note in Commonly Rejected Changes <https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md>:
> 
> Remove support for default: in switch and just use case _:: default is widely used, case _ is too magical, and default is widely precedented in many C family languages.
> I really like to use the switch instead of if case for pattern matching, just because it’s neat block design. I do not want to remove default from switches because it’s a must have and powerful feature.
> 
> I’d like to know why switches must be exhaustive. 
> 
> switch someValue {
>      
> case …:
>     // Do something
>      
> case …:
>     // Do something else
> 
> default:  
>     () // useless nop; do nothing when no pattern matched
> }
> 
> // VS:
> 
> if case … {
>      
> } else if case … {
>      
> } else if case … {
>      
> } // No need for `else`
> Can’t we make default optional, or at least on non-enum values?
> 
> 
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
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